Detection and enumeration of injured bacterial spores in processed foods

Injury of bacterial spores is manifested in ways similar to those for vegetative cells, i.e. altered nutritional requirements, increased sensitivity to inhibitors and modified optimum temperatures. However, injury in spores may involve activation, germination and/or outgrowth systems, in addition to the vegetative growth processes. Food processes in the form of heat, acid, inhibitors, preservatives, low water activity and combinations of these factors can stress spores. Consequently, detection and enumeration methods designed to recover bacterial spores from processed foods must always consider the requirements of injured spores. Injury to germination systems has been circumvented through the addition of non-nutritive agents, such as lysozyme, to recovery media, or by incubation at an alternative temperature which favours a secondary germination system. A shift in temperature has also permitted repair of damaged outgrowth systems. Addition of materials such as starch and charcoal can reduce the sensitivity of injured spores to some inhibitors. Our current research on hypochlorite injury of Cl. botulinum, differing injury responses for B. cereus in rice and TSB, and injury of B. cereus earlier in the heating process at 85 degrees C than at 90 degrees C indicate a need for more information on the injury of spores in processed foods.